1. Field of the Invention
The invention relates to methods for cosmetic, non-structural repair of voids or defects in turbine superalloy components, such as turbine blades and vanes, including service-degraded components. More particularly, the present invention relates to cosmetic, non-structural repair of voids or defects, including cracks, in thermal barrier coated gas turbine blades and vanes with low temperature hardening resins to restore component dimensions at the defect site prior to their recoating with a new thermal barrier coating.
2. Description of the Prior Art
Repair or new fabrication of nickel and cobalt based superalloy material that is used to manufacture turbine components, such as cast turbine blades, is challenging, due to the metallurgic properties of the finished blade material. For example a superalloy having more than 6% aggregate aluminum or titanium content, such as CM247 alloy, is more susceptible to strain age cracking when subjected to high-temperature welding than a lower aluminum-titanium content X-750 superalloy. The finished turbine blade alloys are typically strengthened during post casting heat treatments, which render them difficult to perform subsequent repair. Currently used repair processes for superalloy turbine components by welding or brazing generally require substantial component heating. When a blade constructed of such a material is welded with filler of the same or similar alloy, the blade is susceptible to solidification (aka liquation) cracking within and proximate to the weld, and/or strain age (aka reheat) cracking during subsequent heat treatment processes intended to restore the superalloy original strength and other material properties comparable to a new component.
Non-structural repair or fabrication of metals, including superalloys, is recognized as replacing damaged material with mismatched alloy material of lesser structural property specifications, where the localized original structural performance of the original substrate material is not needed. For example, non-structural or cosmetic repair may be used in order to restore the repaired component's original profile geometry. In the gas turbine repair field an example of cosmetic repair is for filling surface pits, cracks or other voids on a turbine blade airfoil in order to restore its original aerodynamic profile, where the blade's localized exterior surface is not critical for structural integrity of the entire blade. Cosmetic repair or fabrication is often achieved by removing the existing void or defect by grinding or other similar processes to expose fresh unblemished substrate and then filling the ground-out substrate material using oxidation resistant weld or braze alloys of lower strength than the blade body superalloy substrate, but having higher ductility and lower application temperature that does not negatively impact the superalloy substrate's material properties. Grinding out the void or other defect reduces the volume of high-strength superalloy material at the defect site, and merely restores the substrate external profile dimensions by replacement with weaker material.
Diffusion brazing has been utilized to join superalloy components for repair or fabrication by interposing brazing alloy between their abutting surfaces to be joined and heating those components in a furnace (often isolated from ambient air under vacuum or within an inert atmosphere) until the brazing alloy liquefies and diffuses within the substrate of the now-conjoined components. Diffusion brazing can also be used to fill surface defects, such as cracks, in superalloy components by inserting brazing alloy into the defect and heating the component in a furnace to liquefy the brazing alloy and thus fill the crack. In some types of repairs a torch, rather than a furnace can be used as a localized heat source to melt the brazing alloy.
When performing diffusion or torch brazing on superalloy components care must be taken to avoid overheating the substrate and causing its structural degradation, as discussed above. To this end, brazing alloys with relatively low melting points have been used to minimize heating of the overall superalloy substrate. Low melting point brazing alloys often include silicon (Si), boron (B) and/or phosphorous (P) that do not promote good bonding of thermal barrier coating when the brazed blades are recoated for service use.
Superalloy turbine blade and vane braze repair requires expensive and time-consuming braze alloy application as well as post-brazing heat treatment. Those post-repair heat treatment processes risk thermal degradation of the blades or vanes and scrapping of components that are not successfully repaired, wasting all prior repair efforts. Thus for economic reasons, the total repair expense and risk of unsatisfactory blade and vane repair leads to discarding of components where ultimate repair success is questionable. Additionally, as previously noted, current braze repair processes remove strong superalloy substrate material around the repair site and replaces it with structurally weaker material. Effort and expense are undertaken to remove substrate material at the repair site, at least conceptually weakening the remaining substrate. Subsequent post-brazing heat treatment further risks weakening the repaired superalloy component.
Thus, a need exists in the art for a for a method for performing cosmetic repairs on surfaces of superalloy components such as turbine vanes and blades, so that voids, cracks and other surface defects can be repaired, without degrading structural properties of the component substrate.
Another need exists in the art for a method for performing repairs on surfaces of superalloy components, such as turbine vanes and blades, with proven, repeatable repair techniques and repair equipment that do not require removal of substrate material at the repair site, brazing, or post-repair heat treatment procedures that might also degrade structural properties of the component substrate.
Yet another need exists in the art for a method for performing repairs on surfaces of superalloy components, such as turbine vanes and blades, at lower cost, relatively short repair cycle times and higher likely repair success, in order to reduce component repair “fallout” failure and increase the number of components that can be repaired without scrapping them.